In many industrial processes, the water concentration of flowing gas streams must be measured and analyzed with a high degree of speed and accuracy. Such measurement and analysis is required because the water concentration is often critical to the quality of the product produced. Consequently, many complex and sophisticated devices are available for measuring water in gases.
These devices typically incorporate an electrolytic cell. The gas to be measured flows through the cell with a known flow rate. The water concentration of the gas is determined by absorbing the water from the gas, using a hygroscopic film, and electrolyzing the water absorbed in that film. Once equilibrium is achieved, the number of molecules electrolyzed per second, measured as the electrolysis current, is proportional to the number of water molecules entering the cell with the gas each second.
An example of a conventional electrolytic cell is described in U.S. Pat. No. 4,800,000 to D. A. Zatko, incorporated herein by reference. All of the known devices use a plastic, typically an epoxy, filler to mechanically fix the detection unit within the cell. The epoxy also serves as an electrical insulator for the electrodes and as a leak-tight barrier between the entrance and exit of the detection unit (the hollow glass tube) of the cell, so that the sample gas will flow only through and not around the detection unit. Finally, the epoxy provides a leak-tight barrier where the electrical connections penetrate through the metal housing of the cell.
As a result, the plastic packing material is present in the vicinity of both the entrance and exit of the sensing device. These materials are known to be relatively porous and to exchange water with the gas stream. It is known that the absorption and emission properties of materials such as plastics form an obstacle to reaching low water concentrations and fast response times in high purity gas systems.
The use of an epoxy potting compound in the conventional electrolytic cells causes many problems. First, a good bond between the epoxy and the metal wall of the cell housing, on one hand, and between the epoxy and the glass tube, on the other hand, must be assured.
Second, epoxy requires a long curing time, which might be as long as one week at room temperature. If insufficient time is allotted to chemically cure the epoxy, the epoxy will retain moisture. Such retention becomes critical when low moisture level performance tests are attempted just after the cell is manufactured.
Another problem is the disparate thermal coefficient of expansion between the epoxy and both metal and glass. Incompatible expansion consequent upon temperature changes which occur during operation of the cell may cause leakage and cracking of the glass tube.
Moreover, the epoxy material itself and methods of handling the material are not easily reproduced. This problem is the source of fluctuations in the properties of the cells as manufactured. If a cell is rejected because its properties fail to meet specifications, it is desirable to recycle the metal cell body. Such recycling is cumbersome, however, because the epoxy present must be removed.
The epoxy may shrink. If it does shrink, leaks and cavities may form, for example, between the metal housing and the epoxy. Leaks may raise detection limits and cavities may cause slow response times. The outgassing and moisture absorption/desorption properties of epoxy are strongly temperature dependent. Consequently the detection limit and response time of an hygrometer containing a material such as epoxy may become temperature dependent.
Any cracking of the glass tube or short circuit of the electrodes inside the glass tube will be difficult to observe directly once the tube is embedded inside epoxy. In addition, epoxy is a relatively expensive material.
Most of the disadvantages and potential causes of malfunctioning listed above as attributable to the use of epoxy in an hygrometer can be discovered only late in the production process. Often discovery occurs indirectly by observing the negative impact of the disadvantages or malfunctions on the performance of the final cell. Moreover, such discovery requires time consuming tests.